As a request to cut down expenses and time required for a business trip is made in recent years, a call system which holds a teleconference or the like via a communication network such as the Internet has become widely available. A teleconference can be held by use of such call system in which image data and voice data are transmitted/received among a plurality of call terminals once a call has been initiated thereamong.
The enhancement of broadband environment in recent years has also allowed for the transmission/reception of high-quality image data and high-quality voice data among the plurality of call terminals, whereby one can grasp a state of the other party in the teleconference more easily to be able to have a more fulfilling communication by conversation.
Some conferences involve transmission/reception of data that is highly confidential, and thus it is desired to provide a technique which can appropriately restrict a participant to a conference. As a technique to restrict a participant to a conference, Patent Literature 1 has disclosed a technique which, for the purpose of restricting a participant to a conference, registers a participant prior to a conference and restricts the participation by anyone other than those who are registered participants.